Laser gain medium for solid state dye lasers

ABSTRACT

The present invention relates to a laser gain medium comprising at least one active species adapted to be stimulated to emit laser light within a predetermined wavelength range and optical feedback means defining a resonator for said laser light. The feedback means comprise at least one substantially solid cholesteric layer having a substantially planar texture exhibiting selective reflection of light defined by a reflection band tuned to said predetermined wavelength range.

BACKGROUND OF THE INVENTION

The present invention relates to solid state dye lasers and especiallyto a gain medium for solid state dye lasers comprising at least onecholesteric liquid crystalline polymer layer which provides distributedfeedback for laser light emitted in the gain medium.

Laser devices are used in all areas of science and technology togenerate coherent electromagnetic radiation in the infrared, visible orultraviolet region the spectrum, i.e. approximately at wavelengthsbetween 300 and 1800 nm. Lasers operate using the principle of lightamplification by stimulated emission of radiation. If light is incidenton an excited species, they may be stimulated to emit their energy asadditional light with the same frequency, phase, polarisation anddirection as the incident light. As the converse process, i.e.stimulated absorption of unexcited species, is also possible in a givensample, net amplification only occurs when the excited species outnumberthe unexcited species, i.e. when an “inversion” of the atomic levels isestablished. Emission of laser light requires an excitable activespecies, e.g. a dye, which exhibits strong stimulated emission ofradiation. Further, optical feedback means must be provided which act asa resonator for laser light emitted by the active species. The simplestresonator consists of two opposing mirrors arranged on opposite sides ofa medium containing the active species which is conventionally denoted“gain medium”.

Lasers are conventionally classified according to the type of gainmedium used, e.g. gases, dyes, solid state semi conductor, and others.

Among those, dye lasers are of particular interest in many technicalfields because they provide a broad tuneability of the emitted laserlight over the spectral range mentioned above and their pumping methods,i.e. the methods for exciting the active species, are rather flexible.Commonly, excitation of the dye is achieved by means of so-calledoptical pumping using a source of energy such as a flash lamp or a pumplaser. Typical pump lasers are nitrogen, argon iron, and frequencydoubled Nd:YAG (neodymium/yttrium-aluminum-garnet). Dye lasers can beoperated in either continuous-wave (CW) mode with continuous powers oftypically up to 100 mW with very narrow line width or in pulsed modewith energies up to 1 J and pulse durations in the femto seconds range.

While most dye lasers operate with a liquid gain medium, solid-state dyelasers have also been developed, a laser assembly where the laser dyesare incorporated e.g. in a solid polymer matrix such as polymethylmethacrylate (PMMA). Solid state gain media overcome some of thedisadvantages of liquid gain media such as handling problems and healthor environmental hazards associated with many laser dyes and solventscommonly employed.

As an alternative to using end mirrors to define an optical cavity,mirrorless dye lasers with optical feedback distributed throughout thegain medium are known. Distributed feedback is commonly used insemiconductor or dye lasers, especially, when single mode operation isrequired (e.g. Shank et al. “Tunable distributed-feedback dye laser”,Applied Physics Letters, 18, 152 (1971)). The distributed feedback isobtained by a gain medium exhibiting a spatial modulation in opticalproperties such as refractive index or gain in the direction of lightpropagation through the medium. A common method to obtain a periodicmodulation in the gain medium is to interfere two coherent pump laserbeams. Then, the output wave length of stimulated laser light isproportional to the periodicity of the interference pattern. The laseremission wavelength can be tuned e.g. by varying the angle between thepump laser beams.

In U.S. Pat. No. 3,771,065, a liquid gain medium for dye lasers has beensuggested consisting of a laser dye dissolved in a cholesteric liquidcrystal (CLC) which provides distributed feedback. Such gain mediabenefit from special optical properties of the cholesteric or “chiralnematic” phase of certain liquid crystals: CLC's develop a helicalsuperstructure characterized by a local nematic director which isperpendicular to the helix axis but varies linearly with its positionalong the helix axis. The pitch, i.e. the spatial period, is determinedby the concentration and the helical twisting power of the chiralconstituents. As a consequence of the periodicity of the helicalcholesteric structure and the birefringence of the liquid crystal, for arange of wavelengths, light propagation along the helix axis isforbidden for one of the normal modes. Thus incident light of a“forbidden” wavelength is stronly reflected. The edges of this relectionband are at wavelengths which are equal to the refractive indices timesthe helical pitch (c. f. deGennes, “The physics of liquid crystals”,Clarendon Press, Oxford, 1974). Thus, if a dye doped CLC is alignedbetween two glas plates in the so-called planar texture, a “Bragg-type”phase grating is established throughout the CLC layer. Then laseremission is normal to the film plane and the output wavelength is set bythe helical periodicity. By varying the temperature of the CLC host, thehelical pitch of the CLC host can be changed, thereby the outputwavelength of the dye laser can be tuned. Fluid CLC gain media are,however, subject to environmental perturbation, such as temperature, andimpractical for many applications.

In U.S. Pat. No. 6,141,367, the disclosure of which is herebyincorporated by reference into the present application, a solid statedye laser has recently been described which has a solid gain mediumdoped with a fluorescent dye. It has been suggested to use a gain mediumwhich includes a polymeric cholesteric liquid crystal disposed in aplanar texture and frozen into a characteristic wavelength. Theconfiguration of the dye laser including location means which allow forlocating and orienting the gain medium relative to a pump laser areextensively described in this document. However, U.S. Pat. No. 6,141,367does not disclose any specific polymeric CLC which can act as a suitablehost material of a solid state dye laser medium.

Lasing in dye doped cholesteric liquid crystals has been demonstratedfor the first time by Kopp et al. in Opt.Lett. 23, 1709, 1998 and Taheriet al. in ALCOM Symposium on Chirality, Cuyahoga Falls, February 1999.Subsequently, Finkelmann et al. have suggested in Adv. Mater. 2001, 13,No. 14, 1069-1072 to use an elastomeric cholesteric liquid crystal as alaser gain medium. Finkelmann et al. have demonstated that mechanicalstretching of an elastomeric cholesteric liquid crystal allows fortuning the lasing wavelenth. The elastomeric liquid crystal used byFinkelmann et al. comprises a polymeric network synthesized via ahydrosilylation of poly[oxy(methylsilylene] both with an achiralnematogenic monomer, said monomer having a mesogenic group comprising

a chiral cholesterylcarbonate and 1,3,5 triallyloxybenzene as acrosslinking agent. As laser properties such as lasing threshold, linewidth, pulse duration etc. are strongly influenced by the opticalproperties of the distributed feedback gain medium, there is a need forpolymer CLC's which prove particularly suitable for use in a solid statedye laser.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved CLC based gain medium for solid state distributed feedback dyelasers. It is a further object of the present invention to provide alaser gain medium which allows to obtain a high degree of parallelorientation of individual helical axis, particularly when employingindustrial scale production methods. It is still a further object of theinvention to provide a polymeric CLC gain medium which allows for aneasy tuneability of the laser output wavelength.

We have found that these objects are achieved by a laser gain mediumcomprising at least one active species adapted to be stimulated to emitlaser light within a predetermined wavelength range, optical feedbackmeans defining a resonator for said laser light, said feedback meanscomprising at least one substantially solid cholesteric layer having asubstantially planar texture exhibiting selective reflection of lightdefined by a reflection band tuned to said predetermined wavelengthrange, said cholesteric layer being obtained from reactive cholestericmixtures selected from mixtures comprising:

-   -   a) at least one cholesteric, polymerizable monomer; or    -   b) at least one achiral, nematic, polymerizable monomer and one        chiral compound in an inert diluent; or    -   c) at least one cholesteric, crosslinkable oligomer or polymer        selected from the group comprising cholesteric cellulose        derivatives, propargyl-terminated cholesteric polyesters or        polycarbonates, crosslinkable oligo- or polyorgano-siloxanes; or    -   d) crosslinkable cholesteric copolyisocyanates in a        polymerizable diluent; or    -   e) chiral nematic polyesters having flexible chains whose        cholesteric phase can be frozen in by rapid cooling to below the        glass transition temperature,        werein said mixtures b) do not comprise mixtures of an achiral,        nematic, polymerizable monomer having a mesogenic group        comprising        and a chiral cholesterylcarbonate and a crosslinking agent.

While the preferred gain medium of the invention comprises either one ofmixtures a) to e), a suitable gain medium may also comprise mixtures ofmixtures a) to e).

The production of cholesteric layers for gain media according to theinvention offer a range of surprising advantages: The cholesterichelices, particularly of mixtures a) and b), can be oriented withparticular advantage when dilute cholesteric solutions are used.Post-orientation of the cast (as yet unpolymerized and uncrosslinked)layer in order to align the cholesterics is often unnecessary. Thecholesteric layers produced possess an extremely homogeneous layerthickness and can be produced in a reproducible manner. The inventionmakes cost-effective production of solid CLC laser gain media possible.

The cholesteric mixture is preferably applied with a diluent fraction offrom about 5 to 95% by weight, in particular from about 30 to 80% byweight, preferably from about 40 to 70% by weight and, with particularpreference, from about 55 to 60% by weight, based in each case on theoverall weight of the mixture that is to be applied.

The mixtures of the invention may be employed in a pourable form, e.g.mixtures a), b) and c) in an inert diluent and mixture d) in apolymerizable diluent. For a detained description of methods forproducing solid cholesteric polymer layers or films from mixtures a),b), c) and d), reference is made to International Patent application WO99/1173.3 (corresponding to U.S. Ser. No. 09/486,695), the disclosure ofwhich is incorporated herin by reference.

Preferred monomers of group a) are described in DE-A-196 02 848, thefull content of which is incorporated herein by reference. Inparticular, the monomers a) embrace at least one chiral,liquid-crystalline, polymerizable monomer of the formula I[Z¹-Y¹-A¹-Y²-M¹-Y³—]_(n)X   (I)where

-   -   Z¹ is a polymerizable group or a radical which carries a        polymerizable group,    -   Y¹,Y², Y³ independently are chemical bonds, oxygen, sulfur,        —CO—O—,—O—CO—,—O—CO—O—,        —CO—N(R)— or —N(R)—CO—,    -   A¹ is a spacer,    -   M¹ is a mesogenic group,    -   X is an n-valent chiral radical,    -   R is hydrogen or C₁-C₄-alkyl,    -   n is 1 to 6,    -   and Z¹, Y¹, Y^(2,) Y^(3,) A¹ and M¹ can be identical or        different if n is greater than 1.

Preferred radicals Z¹ are:

where each R can be identical or different and is hydrogen orC₁-C₄-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, sec-butyl or tert-butyl. Of the reactive polymerizable groups,the cyanates are able to trimerize spontaneously to cyanurates and aretherefore preferred. Polymerization of the other groups indicatedrequires further compounds having complementary reactive groups.Isocyanates, for example, are able to polymerize with alcohols to giveurethanes and with amines to give urea derivatives. Similar commentsapply to thiiranes and aziridines. Carboxyl groups can be condensed togive polyesters and polyamides. The maleimido group is particularlysuitable for free-radical copolymerization with olefinic compounds suchas styrene. Said complementary reactive groups can be present either ina second compound of the invention, which is mixed with the first, orcan be incorporated into the polymeric network by means of auxiliarycompounds containing 2 or more such complementary groups.

Particularly preferred groups Z¹-Y¹ are acrylate and methacrylate.

Y¹-Y³ can be as defined above, the term a chemical bond meaning a singlecovalent bond.

Suitable spacers A¹ are all groups known for this purpose. The spacerscontain generally 1 or more, e.g. from 2 to 30, preferably 1 to 12 or 2to 12 carbon atoms and consist of linear aliphatic groups. They may beinterrupted in the chain by nonadjacent O, S, NH or NCH₃, for example.Other suitable substituents for the spacer chain are fluorine, chlorine,bromine, cyano, methyl and ethyl.

Examples of representative spacers are:

where

-   -   m is 1 to 3 and    -   p is 1 to 12.

The mesogenic group M¹ preferably has the structure(T-Y⁸)_(s)-Twhere Y⁸ is a bridge in accordance with one of the definitions ofY^(1, s is) 1 to 3 and T is identical or different at each occurrenceand is a divalent isocycloaliphatic, heterocycloaliphatic, isoaromaticor heteroaromatic radical.

The radicals T can also be ring systems substituted by C₁-C₄-alkyl,fluorine, chlorine, bromine, cyano, hydroxyl or nitro. Preferredradicals T are:

Particular preference is given to the following mesogenic groups M¹:

Of the chiral radicals X of the compounds of the formula I particularpreference is given, not least on account of their availability, tothose derived from sugars, from binaphthyl or biphenyl derivatives andfrom optically active glycols, dialcohols or amino acids. In the case ofthe sugars, particular mention should be made of pentoses and hexosesand derivatives thereof.

Examples of radicals X are the following structures, the lines at theend in each case denoting the free valences.

Particular preference is given to

Also suitable, furthermore, are chiral groups having the followingstructures:

Further examples are set out in the German Application P 43 42 280.2.

n is preferably 2.

As preferred monomers of group b), the polymerizable mixture in theprocess of the invention includes at least one achiralliquid-crystalline polymerizable monomer of the formula IIZ²-Y⁴-A²-Y⁵-M²-Y⁶-A³-Y⁷-Z³)_(n)   (II)where

-   -   Z²,Z³ are identical or different polymerizable groups or        radicals which contain a polymerizable group,    -   n is 0 or 1    -   Y^(4,)Y^(5,)Y^(6,)Y⁷ independently are chemical bonds, oxygen,        sulfur,        —CO—O—,—O—CO—,—O—CO—O—,        —CO—N(R) —or —N(R)—CO—,    -   A²,A³ are identical or different spacers and    -   M² is a mesogenic group.

The polymerizable groups, bridges Y⁴ to Y⁷, the spacers and themesogenic groups are subject to the same preferences as thecorresponding variables of the formula I.

The crosslinkable achiral liquid-crystalline monomers, in particularthose which make it possible to prepare liquid-crystalline polymershaving relatively high crosslinking density, may also be compounds ofthe formula

where

-   -   A⁴ and A⁵ are identical or different and are each a        crosslinkable group;    -   the radicals Y⁸ are identical or different, preferably        identical, and are each a single bond, —O —, —S—, —C═N—, —O—CO—,        —CO—O—, —O—CO—O—, —CO—NR—, —NR—CO—, —NR—, —O—CO—NR, —NR—CO—O—,        —CH₂—O—or —NR—CO—NR, in which R is H or C₁-C₄-alkyl; and    -   M³ is a mesogenic group.

These compounds are notable for the fact that they are capable offorming a liquid-crystalline phase and stabilize this phase particularlywell and permanently owing to their increased crosslinkable groupcontent.

Preferably A⁵ is ortho to A⁴ at each occurrence.

Preference is likewise given to compounds where A⁴ and A⁵ are each,independently of one another, a group of the formulaZ-Y⁸-(Sp)_(n)-where

-   -   Z is a crosslinkable radical;    -   Y⁸ is as defined above;    -   Sp is a spacer having from 1 to 30 carbon atoms, in which the        carbon chain may be interrupted by ether oxygen, thioether        sulfur or nonadjacent imino or C₁-C₄-alkylimino groups; and    -   n is 0 or 1.    -   A⁴ and A⁵ are preferably identical.

According to a preferred embodiment Z is selected from:

where the radicals R are each, independently of one another,C₁-C₄-alkyl, for example methyl, ethyl, n- or i-propyl or n-, i- ort-butyl.

According to another preferred embodiment Sp is selected from:

where m is from 1 to 3 and p is from 1 to 12.

According to another preferred embodiment M³ is selected from groups ofthe general formula III:

where

-   -   Y⁸ is as defined above, and    -   Q is substituted or unsubstituted alkylene, such as linear or        branched C₁-C₁₂-alkylene, or a substituted or unsubstituted        aromatic bridging group.

Preferred aromatic bridging groups are selected from

and substituted analogs thereof. Substituted analogs of said bridginggroups can carry from 1 to 4 identical or different substituents peraromatic ring, preferably one or two substituents per ring or perbridging group. Suitable substituents are selected from C₁-C₄-alkyl asdefined above, nitro, halogen, such as F, Cl, Br, I, phenyl orC₁-C₄-alkoxy, the alkyl radical being defined as above.

A detained description of compounds of formula III and methods forproducing such compounds can be found in German patent application DE100 16 524 the disclosure of which is incorporated herein by reference.

In addition to the achiral compounds, the mixture according to b)includes at least one chiral compound. The chiral compound brings aboutthe twisting of the achiral liquid-crystalline phase to form acholesteric phase. In this context, the extent of twisting depends onthe twisting power of the chiral dopant and on its concentration.Consequently, therefore, the pitch of the helix and, in turn, thereflectance wavelength depend on the concentration of the chiral dopant.As a result, it is not possible to indicate a generally validconcentration range for the dopant. The dopant is added in the amount atwhich the cholesteric layer has a reflection band with edges at thedesired lasing wavelength.

Preferred chiral compounds are either those of the formula Ia[Z¹-Y¹-A¹-Y²-M^(a)-Y³—]_(n)X   (Ia),or those of formula IbZ¹-Y¹-A¹-Y²—X²   (Ib),where Z¹, Y¹, Y², Y³, A¹, X and n are as defined above in formula I,M^(a) is a divalent radical which comprises at least one heterocyclic orisocyclic ring system, and X² is a cholesteryl radical or a derivativethereof.

The moiety M^(a) here is similar to the mesogenic groups described,since in this way particularly good compatibility with theliquid-crystalline compound is achieved. M^(a), however, need not bemesogenic, since the compound Ia is intended merely by virtue of itschiral structure to bring about the appropriate twisting of theliquid-crystalline phase. Preferred ring systems present in M^(a) arethe abovementioned structures T, preferred structures M^(a) being thoseof the abovementioned formula (T-Y⁸)_(s)-T. Further monomers and chiralcompounds of group b) are described in WO 97/00600 and its parentDE-A-195 324 08, the full content of which is expressly incorporatedherein by reference.

A preferred moiety X² is a chiral cholesterylcarbonate of formula

Preferred polymers of group c) are cholesteric cellulose derivatives asdescribed in DE-A-197 136 38, especially cholesteric mixed esters of

-   -   (VI) hydroxyalkyl ethers of cellulose with    -   (VII) saturated, aliphatic or aromatic carboxylic acids and    -   (VIII) unsaturated mono- or dicarboxylic acids.

Particular preference is given to mixed esters in which the hydroxyalkylradicals of component VI that are attached by way of ether functions arestraight-chain or branched C₂-C₁₀-hydroxyalkyl radicals, especiallyhydroxypropyl and/or hydroxyethyl radicals. Component VI of the mixedesters of the invention preferably has a molecular weight of from about500 to about 1 million. Preferably, the anhydroglucose units of thecellulose are etherified with hydroxyalkyl radicals in an average molardegree of substitution of from 2 to 4. The hydroxyalkyl groups in thecellulose can be identical or different. Up to 50% of them can also bereplaced by alkyl groups (especially C₁-C₁₀-alkyl groups). One exampleof such a compound is hydroxypropylmethylcellulose.

Compounds which can be used as component VII of the mixed esters thatare employable are straight-chain aliphatic C₁-C₁₀ carboxylic acids,especially C₂-C₆ carboxylic acids, branched aliphatic C₄-C₁₀ carboxylicacids, especially C₄-C₆ carboxylic acids, or straight-chain or branchedhalocarboxylic acids. Component VII can also comprise benzoic acid oraliphatic carboxylic acids with aromatic substituents, especiallyphenylacetic acid. Component VII is with particular preference selectedfrom acetic, propionic, n-butyric, isobutyric and n-valeric acid, inparticular from propionic, 3-chloropropionic, n-butyric and isobutyricacid.

Component VIII is preferably selected from unsaturated C₃-C₁₂ mono- ordicarboxylic acids or monoesters of such dicarboxylic acids, especiallyfrom α, β-ethylenically unsaturated C₃-C₆ mono- or dicarboxylic acids ormonoesters of the dicarboxylic acids.

Component VIII of the mixed esters that are employable is withparticular preference selected from acrylic, methacrylic, crotonic,vinylacetic, maleic, fumaric and undecenoic acid, especially fromacrylic and methacrylic acid.

Component VI is preferably esterified with component VII and VIII in anaverage molar degree of substitution of from 1.5 to 3, in particularfrom 1.6 to 2.7 and with particular preference, from 2.3 to 2.6.Preferably about 1 to 30%, in particular from 1 to 20% or 1 to 10%, withparticular preference, from about 5 to 7% of the OH groups of componentVI are esterified with component VIII.

The proportion of component VII to component VIII determines the hue ofthe polymer.

Highly suitable polymers of group c), moreover, are thepropargyl-terminated cholesteric polyesters or polycarbonates describedin DE-A-197 17 371.

It is also possible to employ crosslinkable oligo- orpolyorgano-siloxanes, as are known, for example, from EP-A-0 358 208,DE-A-195 41 820 or DB-A-196 19 460.

Among these compounds preference is given to polyesters orpolycarbonates having at least one propargyl end group of the formulaR³C≡C—CH₂—, where R³ is H, C₁-C₄-alkyl, aryl or Ar—C₁-C₄-alkyl (eg.benzyl or phenethyl) which is attached to the polyesters orpolycarbonates directly or via a linker. The linker is preferablyselected from

-   -   (the propargyl group is attached to X),        where R⁴ is H, C₁-C₄-alkyl or phenyl, X is O, S or NR² and R² is        H, C₁-C₄-alkyl or phenyl.

In the polyesters, the propargyl end group is preferably attached by wayof

The polyesters preferably comprise

-   -   (IX) at least one aromatic or araliphatic dicarboxylic acid unit        and/or at least one aromatic or araliphatic hydroxycarboxylic        acid unit, and    -   (X) at least one diol unit.

Preferred dicarboxylic acid units are those of the formula

especially those of the formula

where each of the phenyls or the naphthyl can carry 1, 2 or 3substituents selected independently from C₁-C₄-alkyl, C₁-C₄-alkoxy,halogen or phenyl, and where

-   -   W is NR, S, O, (CH₂)_(p)O(CH₂)_(q), (CH₂)_(m) or a single bond,    -   R is alkyl or hydrogen,    -   m is an integer from 1 to 15, and    -   p and q independently are integers from 0 to 10.

Preferred hydroxycarboxylic acid units are those of the formula

where each phenyl or the naphthyl can carry 1, 2 or 3 substituentsselected independently from C₁-C₄-alkyl, C₁-C₄-alkoxy, halogen orphenyl.

Preferred diol units are those of the formula

especially those of the formula

where

-   -   L is alkyl, alkoxy, halogen, COOR, OCOR, CONHR or NHCOR,    -   X is S, O, N, CH₂ or a single bond,    -   A is a single bond, (CH₂)_(n), O(CH₂)_(n), S(CH₂)_(n),        NR(CH₂)_(n),    -   R is alkyl or hydrogen,    -   R¹ is hydrogen, halogen, alkyl or phenyl, and    -   n is an integer from 1 to 15.

Preference is given to polyesters comprising at least one dicarboxylicacid unit of the formula

and at least one diol unit of the formula

where R³ is H, halogen, C₁-C₄-alkyl, especially CH₃ or C(CH₃)₃, or isphenyl.

Further preferred compounds are diesters of the formulaP—Y—B—CO—O-A-O—CO—B—Y—P, where P is a propargyl end group of theabove-defined formula, Y is O, S or NR² (R²═C₁-C₄-alkyl), B is

where each phenyl or the naphthyl can carry 1, 2 or 3 substituentsselected independently from C₁-C₄-alkyl, C₁-C₄-alkoxy, halogen orphenyl, and A (together with the adjacent oxygens) is one of theabovementioned diol units.

Particularly preferred diesters are those of the above formula in whichB is

and especially diesters of the formula

-   -   A is as defined under XI.

Further preferred compounds are polycarbonates comprising at least oneincorporated diol unit of the above formulae,

-   -   especially of the formulae

Preference is given here to those polycarbonates which comprise as diolunits at least one mesogenic unit of the formula

at least one chiral unit of the formula

and, if desired, a nonchiral unit of the formula

where R¹ is as defined above and in particular is H or CH₃.

Particularly preferred polycarbonates are those having propargyl endgroups of the formula HC≡CCH₂O—R⁵—CO, in which R⁵ is

Further suitable polymers of group c) are cholesteric polycarbonatescontaining photoreactive groups even in a non-terminal position. Suchpolycarbonates are described in DE-A-196 31 658 and are preferably ofthe formula XIII

where the molar ratio w/x/y/z is from about 1 to 20/from about 1 to5/from about 0 to 10/from about 0 to 10. Particular preference is givento a molar ratio w/x/y/z of from about 1 to 5/from about 1 to 2/fromabout 0 to 5/from about 0 to 5.

In the formula XIII

-   -   A is a mesogenic group of the formula    -   B is a chiral group of the formula    -   D is a photoreactive group of the formula        and    -   E is a further, nonchiral group of the formula        where    -   L is alkyl, alkoxy, halogen, COOR, OCOR, CONHR or NHCOR,    -   X is S, O, N, CH₂ or a single bond,    -   R is alkyl or hydrogen,    -   A is a single bond, (CH₂)_(n), O(CH₂)_(n), S(CU₂)_(n),        NR(CH₂)_(n),    -   R¹ is hydrogen, halogen, alkyl or phenyl, and    -   n is an integer from 1 to 15.

If R¹ is alkyl or halogen and A is a single bond or if R¹ is H or alkyland A is

the groups concerned are solubility-enhancing groups. Examples of theseare

Isosorbide, isomannide and/or isoidide is the preferred chiralcomponent.

The proportion of the chiral diol structural units is preferably withinthe range from 1 to 80 mol-% of the overall content of diol structuralunits, with particular preference from 2 to 20 mol-%, depending on thedesired interference hue.

Examples of preferred polymers of group d) are crosslinkable cholestericcopolyisocyanates as described in U.S. Pat. No. 08,834,745. Suchcopolyisocyanates feature repeating units of the formulae

and if desired of the formula

where

-   -   R¹ is a chiral aliphatic or aromatic radical,    -   R² is a crosslinkable radical and    -   R³ is an achiral radical.

Unless stated otherwise, alkyl is to be understood here (both alone andin definitions such as alkoxy, dialkyl, alkylthio, etc.) as branched andunbranched C₁-C₁₂-alkyl, preferably C₃-C₁₂-, with particular preferenceC₄-C₁₀- and, in particular, C₆-C₁₀-alkyl.

R¹ is preferably selected from (chiral) branched or unbranched alkyl,alkoxyalkyl, alkylthioalkyl, cycloalkyl, alkylphenyl or C₃-C₉-epoxyalkylradicals or radicals from esters of C₁-C₆ fatty acids withC₁-C₆-alkanols or C₃-C₉-dialkyl ketones. The ester radical may beattached to the nitrogen either via the fatty acid moiety or via thealkanol residue. The radical R¹ may have 1, 2 or 3 substituents, whichare identical or different and are selected from alkoxy,di-C₁-C₄-alkylamino, CN or C₁-C₄-alkylthio groups or halogen atoms.

R¹ is preferably selected from alkyl, alkoxyalkyl, radicals from estersof C₁-C₆ fatty acids with C₁-C₆-alkanols, C₃-C₉-dialkyl ketones andepoxidized C₃-C₉-epoxyalkyl radicals, where R¹ may be substituted by 1or 2 radicals which are identical or different and are selected fromalkoxy, halogen, CN and CF₃. Preferred substituents of branched orunbranched alkyl or alkoxy radicals are selected from alkoxy groups,halogen atoms and CN; from esters of C₁-C₆ fatty acids withC₁-C₆-alkanols, from alkoxy groups, halogen atoms, CN and CF₃; and, forC₃-C₉-dialkyl ketones, from alkoxy groups, halogen atoms and CN.

The main chain of the radical R¹ has, in particular, a length of from 3to 12, especially 6 to 10, preferably 6 to 8 members (carbons, oxygensand/or sulfurs). Particularly preferred radicals R¹ are selected from

With very particular preference, component III of the copolyisocyanatesthat can be employed is derived from 2,6-dimethylheptyl isocyanate.

The radical R² of the copolyisocyanates that can be employed ispreferably selected from C₃-C₁₁-alkenyl radicals, C₄-C₁₁-vinyl etherradicals (=vinyl C₂-C₉-alkyl ethers), ethylenically unsaturated C₃-C₁₁carboxylic acid radicals and esters of ethylenically unsaturated C₃-C₆monocarboxylic acids with C₂-C₆-alkanols, the linkage to the nitrogentaking place via the alkanol residue of the ester. The radical is withparticular preference selected from methyl acrylate, ethyl acrylate,propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexylacrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate,n-butyl methacrylate, isobutyl methacrylate, and 2-ethylhexylmethacrylate, especially from ethyl acrylate or ethyl methacrylate.

The radical R³ preferably has the same definitions as the radical R¹.However, it is achiral, i.e. it has no center of chirality or is in theform of a racemic mixture.

With particular preference, the main chain of the radical R³ has alength of from 4 to 12, in particular 6 to 10, preferably 6 to 8 members(carbons, oxygens and/or sulfurs). Component V of the copolyisocyanatesof the invention is, with very particular preference, derived fromn-hexyl isocyanate, n-heptyl isocyanate or n-octyl isocyanate.

Components III, IV and V are preferably present in a molar ratioIII:IV:V of about 1 to 20:1 to 20:50 to 98, in particular about 5 to15:5 to 15:65 to 90, and, with particular preference, about 15:10:75.

The units III, IV and V can be distributed randomly in thecopolyisocyanates which can be employed.

Suitable polymers of group e) are chiral nematic polyesters havingflexible chains and comprising isosorbide, isomannide and/or isoidideunits, preferably isosorbide units, and also comprising at least onechain-flexibilizing unit selected from (and derived from)

-   -   (a) aliphatic dicarboxylic acids,    -   (b) aromatic dicarboxylic acids with a flexible spacer,    -   (c) α,ω-alkanoids,    -   (d) diphenols with a flexible spacer, and    -   (e) condensation products of a polyalkylene terephthalate or        polyalkylene naphthalate with an acylated diphenol and with an        acylated isosorbide,    -   as are described in DE-A-197 04 506.

The polyesters are noncrystalline and form stable Grandjean textureswhich can be frozen in on cooling to below the glass transitiontemperature. The glass transition temperatures of the polyesters, inturn, are despite the flexibilization above 80° C., preferably above 90°C. and, in particular, above 100° C.

The polyesters that can be employed include as units (a) preferablythose of the formula—OC—(CH₂)_(n)—CO—where n is from 3 to 15, especially 4 to 12, and with particularpreference adipic acid;

-   -   as units (b) preferably those of the formula        where    -   A is (CH₂)_(n), O(CH₂)_(n)O or (CH₂)_(o)—O—(CH₂)_(p),    -   n is from 3 to 15, especially 4 to 12, with particular        preference 4 to 10, and    -   o and p independently are from 1 to 7;    -   as units (c) preferably those of the formula        —O—(CH₂)_(n)—O— or —O—(CH₂—CH₂—O)_(m)—,        where    -   n is from 3 to 15, especially 4 to 12, with particular        preference 4 to 10, and    -   m is from 1 to 10; and    -   as units (d) preferably those of the formula        where    -   A is (CH₂)_(n), O(CH₂)_(n)O or (CH₂)_(o)—O—(CH₂)_(p),    -   n is from 3 to 15, especially 4 to 12, with particular        preference 4 to 10, and    -   o and p independently are from 1 to 7.

The polyesters that can be employed additionally comprise, asnonflexible acid component, preferably dicarboxylic acid units of theformula

and as nonflexible alcohol component diol units of the formula

where

-   -   L is alkyl, alkoxy, halogen, COOR, OCOR, CONHR or NHCOR,    -   X is S, O, N, CH₂ or a single bond,    -   A is a single bond,        where    -   R¹ is hydrogen, halogen, alkyl or phenyl and    -   R is alkyl or hydrogen.

If desired, the polyesters that can be employed comprise additionalflexible diol units of the formula

where

-   -   R¹ is hydrogen, halogen, alkyl or phenyl,    -   A is (CH₂)_(n), O(CH₂)_(n), S(CH₂)_(n) or NR(CH₂)_(n) and    -   n is from 1 to 15.

Very particular preference is given, in accordance with the invention,to the use of chiral compounds and nematic monomers of group b),especially of chiral compounds of the formula 2:

or of the formula 5:

and nematic monomers of the formula 1:

or preferably of the formula 3:

or with particular preference, of the formula 4:

where n₁ and n₂ in formulae 1 and 3 are independently 4 or 6, R′ in theformula 1 is H or Cl and the monomers of the formula 1 or 3 arepreferably employed as mixtures of the compounds with n₁/n₂=4/4, 4/6,6/4 or 6/6, and R in formula 4 is H, Cl or CH₃. It is also possible inaccordance with the invention, however, to employ other cholestericmixtures, examples being the mixtures disclosed in EP-A-686 674.

Gain media may be prepared from solutions using inert diluents such as,e.g. for mixtures a) or b) linear or branched esters, especially aceticesters, cyclic ethers and esters, alcohols, lactones, aliphatic andaromatic hydrocarbons, such as toluene, xylene and cyclohexane, and alsoketones, amides, N-alkylpyrrolidones, especially N-methylpyrrolidone,and in particular tetrahydrofuran (THF), dioxane and methyl ethyl ketone(MEK).

According to a preferred embodiment of the invention said cholestericmixtures further comprise at least one crosslinking agent. Examples ofsuitable cross linking agents include as described in U.S. Pat. No.08,834,745. Examples of such crosslinking agents are

-   -   esters of α,β-unsaturated mono- or dicarboxylic acids,        especially C₃-C₆ mono- or dicarboxylic acids, with        C₁-C₁₂-alkanols, C₂-C₁₂-alkanediols or their C₁-C₆-alkyl ethers        and phenylethers, for example the acrylates and methacrylates,        hydroxyethyl or hydroxypropyl acrylate or methacrylate, and        2-ethoxyethyl acrylate or methacrylate;    -   vinyl C₁-C₁₂-alkyl ethers, such as vinyl ethyl, vinyl hexyl or        vinyl octyl ether;    -   vinylesters of C₁-C₁₂ carboxylic acids, such as vinyl acetate,        vinyl propionate, vinyl laurate;    -   C₃-C₉ epoxides, such as 1,2-butylene oxide, styrene oxide;    -   N-vinylpyrrolidone, N-vinylcaprolactam, N-vinylformamide;    -   vinylaromatic compounds, such as styrene, α-methylstyrene,        chlorostyrene, and    -   compounds having two or more, preferably at least three        polymerizable ethylenically unsaturated double bonds, such as        di- or triesters of di- or triols (including polyethylene        glycols) with acrylic or methacrylic acid; mono-, di- or        triesters of α,β-unsaturated mono-, di- or tricarboxylic acids        with vinyl or allyl or methallyl alcohol; di- or triethers of        di- or triols with vinyl or allyl or methallyl alcohol; or di-        or trivinylbenzene, or di- or triallylbenzene, or di- or        trivinyloxybenzene, or di- or triallyloxybenzene, or        N-triallylisocyanurate.

Examples of preferred crosslinking agents are 2-ethoxyethyl acrylate,diethylene glycol diacrylate, ethylene glycol dimethacrylate, diethyleneglycol dimethacrylate, triethylene glycol dimethacrylate, trimethylolpropane di(meth)acrylate or tri(meth)acrylate, diethylene glycolmonomethyl ether acrylate, phenoxyethyl acrylate,5-acryloxymethyl-1,2-dioxane, tetrahydrofurfuryl acrylate, tetraethyleneglycol dimethacrylate, or particularly preferred, styrene and 1,3,5triallyloxybenzene.

If desired, water can also be added to the diluent, or can even beemployed as the sole diluent. Water is the preferred diluent, if thegain medium of the invention is prepared from emulsion.

For photochemical polymerization, the crosslinkable or polymerizablemixture may include customary commercial photoinitiators. For curing byelectron beams, such initiators are not required. Examples of suitablephotoinitiators are isobutyl benzoin ether,2,4,6-trimethylbenzoyldiphenylphosphine oxide, 1-hydroxycyclohexylphenyl ketone,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)furan-1-one, mixtures ofbenzophenone and 1-hydroxycyclohexyl phenyl ketone,2,2-dimethoxy-2-phenylacetophenone, perfluorinated diphenyltitanocenes,2-methyl-1-(4-[methylthio]phenyl)-2-(4-morpholinyl)-1-propanone,2-hydroxy-2-methyl-1-phenylpropan-1-one, 4-(2-hydroxyethoxy)phenyl2-hydroxy-2-propyl ketone, 2,2-diethoxyacetophenone,4-benzoyl-4′-methyldiphenyl sulfide, ethyl 4-(dimethylamino)benzoate,mixtures of 2-isopropylthioxanthone and 4-isopropylthioxanthone,2-(dimethylamino)ethyl benzoate, d,1-camphorquinone,ethyl-d,1-camphorquinone, mixtures of benzophenone and4-methylbenzophenone, benzophenone, 4,4′-bisdimethylaminobenzophenone,(η⁵-cyclopentadienyl) (η⁶-isopropylphenyl) iron(II) hexafluorophosphate,triphenylsulfonium hexafluorophosphate or mixtures of triphenylsulfoniumsalts, and also butanedioldi-acrylate, dipropylene glycol diacrylate,hexanediol diacrylate, 4-(1,1-dimethylethyl)cyclohexyl acrylate,trimethylolpropane triacrylate and tripropylene glycol diacrylate.

According to one preferred embodiment of the present invention, theliquid crystal polymer is an elastomeric material. It has been foundthat cholesteric liquid crystal elastomers exhibit a spontaneous anduniform orientation of the helical structure. Such materials do not needany mechanical support for a stable orientation of the helicalstructure. One particular advantage of cholesteric liquid crystalelastomers resides in the fact that the helical structure can bemodified by external mechanical fields. Consequently laser emission canbe tuned by external mechanical deformations. The elastomeric propertiesof the gain medium are essentially controlled by the concentration of acrosslinking agent and/or by the polymerization conditions.

According to one embodiment of the invention, the laser gain medium iscomprised of at least three layers, with the active species, e.g. thelaser dye, being disposed between a adjacent cholesteric layers actingas optical feedback means. Thus, laser light generated in theintermediate layer can freely propagate throughout the intermediatelayer and is subsequently reflected in the adjacent cholesteric layers.Similar arrangements are denoted “distributed Bragg reflectors (DBR)” indiode laser assemblies. The intermediate layer may for instance be apolymer layer doped with a suitable laser dye or any other active mediumwhich is readily coatable with cholesteric layers. It is known that thereflectance of cholesteric layers depend on the layer thickness. Thus,one cholesteric layer may be adapted to provide for almost 100%reflectance while the other cholesteric layer may be less reflective inorder to obtain a directional output of the laser beam.

According to another embodiment of the invention, the active species maybe comprised in the cholesteric layer. Thus, in one variant of theinvention, the gain medium merely consists of a single cholesteric layerhaving a suitable laser dye dissolved therein. Such a gain media can beprepared in various ways. E.g., a solution comprising theabove-mentioned monomers and a laser dye may be applied to a substrateand polymerized after evaporation of the solvent. Laser dyes may also beadded to the polymerized cholesteric layer, e.g. by swelling thecholesteric polymer with a dye solution and subsequent evaporation ofthe solvent.

In another variant, the active species can be covalently bound to acompound of the cholesteric layer. The laser dye may, for instance, be asuitable reactive monomer co-polymerized with nematic and achiralcompounds. The laser dye can also be, for instance, a side group of oneof the monomeric compounds forming the cholesteric layer.

Rather than a conventional laser dye, the active species may also be apolymeric material capable of exhibiting stimulated emission of light.Conjugated polymers such as poly(p-phenylene vinylene) as described e.g.by Kallinger et al. in Adv. Mater. 1998, 10, pages 920 et seq. arepreferred active species for gain media of the present invention.Preferably, a layer of the conjugated polymer material is prepared andcholesteric reflective layers are disposed on both sides of theconjugated polymer layer.

However, due to a large variety of available compounds, laser dyes arethe preferred active species of the present invention.

Suitable laser dyes are described in Uhlmann's Encyclopedia ofIndustrial Chemistry, 50th edition , volume A 15, pages 151 et seq.Suitable dyes comprise polyphenyl and heteroaromatic compounds,stilbenes, coumarins, xanthene or methine dyes.

It is known that the reflectance band of a cholesteric layer is bound byedges at λ_(e)=n_(e) P and λ₀=n₀ P, with n_(e) and n₀ being therefractive indices, respectively along and perpendicular to themolecular director and P being the pitch of the helical structure. Withthe reflectance band of the cholesteric layer being tuned to thefluorescence emission spectrum of the dye, below lasing threshold, analteration of the emission spectrum and an enhancement of emission atthe reflection band edges is observed. Light enission from dye moleculesexperiences a distributed feedback at the periodicity of n₀ P=λ₀. Thusabove lasing threshold, a narrow lasing line is observed essentially ator close to λ₀.

Consequently, in the gain medium of the present invention, the activespecies can be stimulated to emit laser light within a predeterminedwavelength range corresponding essentially to its fluorescence emissionspectrum. The cholesteric layer is selected such that the edge of itsreflection band falls within the emission spectrum of the activespecies.

The present invention is also directed to a solid state dye lasercomprising a laser gain medium as described above and pump means adaptedto excite said laser gain medium. Preferred pump means are optical pumpmeans such as flash lights or pump lasers. Any source capable ofemitting electromagnetic radiation with a frequency higher than thelaser emission frequency will be a suitable pump source. Preferred pumplasers include continuous wave lasers, such as argon or krypton lasers,or puls lasers such as nitrogen or excimer lasers or frequency doubledNd:YAG lasers.

The above and other features and advantages of this invention will bemore readily apparent from a reading of the following detaileddescription of various aspects of the invention taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a preferred embodiment of solid-state dyelaser system of the present invention.

FIG. 2 depicts a schematic view of a first embodidment of the gainmedium of the invention.

FIG. 3 depicts a variant of the gain medium of FIG. 2.

FIG. 4 depicts a schematic view of a second embodidment of the gainmedium of the invention.

FIG. 5 depicts a variant of the gain medium of FIG. 4.

DETAILED DESCRIPTION PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a preferred embodiment of the solid state dye laser of thepresent invention. The solid state dye laser comprises pump means 10,for instance frequency doubled mode-locked Nd:YAG laser. Suitable means11 to control the pulse energy of the pump laser 10, e.g. a polarizer,half-wave plate assembly, maybe provided. Optical means, e.g. mirrors12, 13 and a lense assembly 14, are used to focus the pump beam 15 onthe gain medium 16 of the present invention. A collector lens 17 is usedto collect the emitted laser light, e.g. in fiber optics wave guide 18.As can be seen, conventional longitudinal optical pumping or “endpumping” is employed, i.e. the pump laser beam is incident along thelength of the gain medium parallel to the propagation direction of thelaser emission. The dye-doped gain medium is typically 5-100 micronthick and dye concentration in the gain medium is typically 10⁻⁴ to 10⁻²molar.

In FIG. 2, a cross-sectional view of first embodiment of the gain medium16 of the invention is depicted. The gain medium consists of a singledye-doped cholesteric layer 20. Longitudinal optical pumping isemployed. The pump laser beam 15 and the dye laser beam 17 travelco-linearly within the laser gain medium 20 as shown by the arrows.Without further modification, the dye laser beam is emitted from bothsurfaces 21, 22 of the cholesteric layer 20.

In FIG. 3, a variant of the gain medium of FIG. 2 is depicted. Surface21 of gain medium 20 is coated with a high reflectance dichroic coating23 that reflects substantially at dye laser wavelength but transmits atpump laser wavelengths. Surface 22 of gain medium 20 is coated with ahigh-reflectance dichroic coating 24 that reflects substantially at pumpwave length but transmits at dye laser wave length. Thus, the specificarrangement of FIG. 3 improves a directional emission of dye laser lightwhich can only exit gain medium 20 via surface 22.

The embodiments of FIGS. 4 and 5 correspond to the embodiments of FIGS.2 and 3, respectively, except that gain medium 20 is comprised of threelayers, namely an intermedium active polymeric layer 25 which hosts thelaser dye but which itself does not exhibit distributed feedback. Layer25 is interposed between two cholesteric layers 26, 27 which act asdistributed reflectors for laser light emitted in the active layer 25.Reflectance of layer 27 is, however reduced as compared to thereflectance of layer 26 thus providing for a directional output of a dyelaser beam 17.

Directional output of laser light may be further improved if additionalreflective coatings 23, 24 are employed as described in FIG. 5.Reflective coatings 23 and 24 correspond to the reflective coatings 23,24 of the embodiment of FIG. 3.

EXAMPLE 1

A cholesteric liquid crystalline elastomeric layer was prepared asfollows: A network was synthesized via a hydrosilylation reaction of 100mol/% poly[oxv(methylsilylene)] with both 72 mol/% of an achiralnematogenic monomer of formula

18 mol/% of a chiral cholesterylcarbonate of formula

and 10 mol/% of a crosslinking agent of formula

p can be either 0 or 1.

The synthesis consists of two steps, which result in an overall oblatechain conformation that causes a uniform orientation of the helix axisin the network, Firstly, a weakly crosslinked gel is synthesized; thecrosslinking reaction is incomplete at this stage. The statisticallyaveraged network chain conformation is assumed to be spherical here.Secondly, an anisotropic deswelling procedure is implemented, where thegel, swollen with the solvent toluene, is allowed to deswell only in onedirection, while keeping the dimensions in the two perpendiculardirections to this constant. This anisotopic deswelling is equivalent toa uniaxial compression (or biaxial stretch), which gives rise to theoblate chain conformation. Due to the coupling between network chainconformation and liquid crystalline order, the helix axis becomesaligned perpendicular to the directions along which he dimensions wereheld constant during the deswelling. Under these conditions, thecrosslinking reaction is completed, resulting in a highly orderedmacroscopic cholesteric liquid single crystal elastomer. To achievelasing, the laser dye DCM(4-(dicyanomethylene)-2-methyl-6-(4-dimethylamino styryl)-4 H-pyran) isdissolved in the cholesteric layer. In order to dope the elastomer withpurified DCM, the network is simply swollen in toluene containing thedye and subsequently dried. The dye conentration in loluene was suchthat when the solvent was removed. 0.2 wt/% of DCM remained dissolved inthe cholesteric layer. With the above concentration of the chiralcomponent, the network shows a selective reflection of left circularlypolarized light with reflection maximum at λ_(R)=590 nm.

To carry out the lasing experiment, a free standing dyed cholestericlayer is corner mounted on the movable faces of a four-jaw chuck over anoptical port, as shown in FIG. 6. The pump beam is a frequency doubledmode-locked Nd:YAG laser with 35 ps pulses at a wavelength of λ=532 nm .The experimental setup corresponds to the schematic drawing of FIG. 1.

The pulse energy was controlled with a polarizer and half-wave plate.The pump beam was focused on the sample using a f=20 cm, 2.5 cm diameterlens, the beam diameter at the sample was 300 μm. The emitted light wascollected and focused to the entrance slit of a TRIAX 550 (JovinYvon-Spex) spectrometer, The emission was recorded with an i-SpectrumOne intensified CCD (JovinYvon-Spex) detector, operated in thecontinuous mode. The samples showed,

1. A laser gain medium comprising at least one active species adapted tobe stimulated to emit laser light within a predetermined wavelengthrange, optical feedback means defining a resonator for said laser light,said feedback means comprising at least one substantially solidcholesteric layer having a substantially planar texture exhibitingselective reflection of light defined by a reflection band tuned to saidpredetermined wavelength range, said cholesteric layer being obtainedfrom reactive cholesteric mixtures selected from mixtures comprising: a)at least one cholesteric, polymerizable monomer; or b) at least oneachiral, nematic, polymerizable monomer and one chiral compound in aninert diluent; or c) at least one cholesteric, crosslinkable oligomer orpolymer selected from the group comprising cholesteric cellulosederivatives, propargyl-terminated cholesteric polyesters orpolycarbonates, crosslinkable oligo- or polyorgano-siloxanes; or d)crosslinkable cholesteric copolyisocyanates in a polymerizable diluent;or e) chiral nematic polyesters having flexible chains whose cholestericphase can be frozen in by rapid cooling to below the glass transitiontemperature. werein said mixtures b) do not comprise mixtures of anachiral, nematic, polymerizable monomer having a mesogenic groupcomprising

and a chiral cholesterylcarbonate and a crosslinking agent.
 2. The lasergain medium of claim 1, wherein said monomer of mixture a) is a chiral,liquid-crystalline, polymerizable monomer of the formula I[Z¹-Y¹-A¹-Y²-M¹-Y³]_(n)X   (I) where Z¹ is a polymerizable group or aradical which carries a polymerizable group, Y¹, Y², Y³ independentlyare chemical bonds, oxygen, sulfur,—CO—O—, —O—CO—, —O—CO—O—,—CO—N(R)— or —N(R)—CO—, A¹ is a spacer, M¹ is a mesogenic group, X is ann-valent chiral radical, R is hydrogen or C₁-C₄-alkyl, n is 1 to6, andZ¹, Y¹, Y², Y³, A¹ and M¹ can be identical or different if n is greaterthan
 1. 3. The laser gain medium of claim 1, wherein said cholestericmixture b) comprises at least one achiral, nematic, polymerizablemonomer of the formula IIZ²-Y⁴-A²-Y⁵-M²-Y⁶-A³)-Y⁷-Z³)_(n)   (III) where Z², Z³ are identical ordifferent polymerizable groups or radicals which contain a polymerizablegroup, n is 0 or 1 Y⁴, Y⁵, Y⁶, Y⁷ independently are chemical bonds,oxygen, sulfur,—CO—O—, —O—CO—, —O—CO—O—,—CO—N(R)— or —N(R)—CO—, A², A³ are identical or different spacers and M²is a mesogenic group, and at least one chiral compound of formula Ia[Z¹-Y¹-A¹-Y²-M^(a)-Y³—]_(n)X   (Ia), or of formula IbZ¹-Y¹-A¹-Y²—X²   (Ib), where Z¹, Y¹, Y^(2,) Y^(3 ,) A¹, X and n are asdefined in claim 2, M^(a) is a divalent radical which comprises at leastone heterocyclic or isocyclic ring system, and X² is a cholesterylradical or a derivative thereof.
 4. The laser gain medium of claim 1,wherein said cholesteric mixtures further comprise at least onecrosslinking agent.
 5. The laser gain medium of claim 1, wherein saidcholesteric liquid crystal polymer is an elastomer.
 6. The laser gainmedium of claim 1, wherein at least one intermediate layer comprisingsaid active species is disposed between adjacent layers of saidcholesteric layers.
 7. The laser gain medium of claim 1, wherein saidactive species is comprised in said cholesteric layer.
 8. The laser gainmedium of claim 7, wherein said active species is dissolved in saidcholesteric layer.
 9. The laser gain medium of claim 7, wherein saidactive species is bound to a compound of said cholesteric layer.
 10. Thelaser gain medium of claim 1, wherein said active species is aconjugated polymer.
 11. The laser gain medium of claim 1, wherein saidactive species is a laser dye.